H7N9

Scientists taking a first look at the genetics of a bird flu strain that has killed three people in China said Wednesday that the virus could be harder to track than its better-known cousin H5N1 because it might be able to spread among poultry without showing any signs.

China reported Tuesday that four more people in one province have been seriously sickened by a bird flu virus new to humans, as cities along the eastern seaboard stepped up public health measures to guard against the disease, which has already caused two deaths.